Structure of liquid ammonia at high pressures and temperatures

Abstract

The structure of liquid ammonia (${\mathrm{NH}}_{3}$) is investigated from 1 to 6.3 GPa and up to 800 K by means of synchrotron x-ray diffraction (XRD) and ab initio molecular dynamics (AIMD) simulations. The XRD data are used to extract the molecular structure factor ${S}_{\mathrm{mol}}(Q)$, pair distribution function (PDF) ${g}_{\mathrm{mol}}(r)$, and the density of ${\mathrm{NH}}_{3}$. There is an excellent agreement between present ${S}_{\mathrm{mol}}(Q)$ and ${g}_{\mathrm{mol}}(r)$ at our lowest density and those reported in reference neutron experiments. Our densities agree better with the equation of state (EoS) of Tillner-Roth et al. [DKV-Tagungsbericht (Hannover, New York, 1993), Vol. 20, p. 167] than with more recent EoS models. The experimental structure factor and PDF are well reproduced by AIMD simulations using either the Becke-Lee-Yang-Parr or the Perdew-Burke-Ernzerhof exchange-correlation functional. The shapes of ${S}_{\mathrm{mol}}(Q)$ and ${g}_{\mathrm{mol}}(r)$ vary little over the investigated pressure range and suggest a compact liquid with weak orientational correlations between molecules, which is corroborated by the coordination number varying from 12.7 to $\ensuremath{\sim}14$. The simulations are used to study the evolution of the site-site pair distribution functions, which reveals that the number of H bonds per molecule (between 1.5 and 2) do not evolve with density and that the distribution of H atoms around N atoms becomes more and more anisotropic with pressure.